TEMPERATURE-DEPENDENCE OF EXCITATION TRANSFER IN LH2 OF RHODOBACTER-SPHAEROIDES

Using two-color pump-probe femtosecond spectroscopy, the temperature d ependence of the energy transfer rate within the peripheral light-harv esting antenna (LH2) of the photosynthetic bacterium Rhodobacter sphae roides has been measured. The energy transfer time from B800 to B850 i s determined to be 0.7, 1.2, and 1.5 ps at 300, 77, and 4.2 It, respec tively, These data, combined with earlier results, have been analyzed with regard to the crystal structure and spectroscopic properties of t he purple bacterial LH2 complex. We conclude that the transfer within B800 occurs mainly via the incoherent Forster hopping mechanism. For B 800 to B850 transfer, estimates based on the Forster formula yield con siderably slower transfer times than experimentally observed,suggestin g that an additional mechanism may be involved in enhancing the transf er rate. We suggest two possibilities: transfer via the upper excitoni c component of B850 band and/or transfer mediated by a carotenoid mole cule.